1. Field of the Invention
The present invention pertains to a relay lens system, and more particularly to a relay lens system suitable for attachment to a video camera or a still video camera, that re-forms an image on, e.g., an electronic imaging device, etc., as a second image, a first image being formed by means of an image forming lens.
2. Description of the Related Art
Video cameras and still video cameras (SVCs) that convert images into digital signals for each pixel using a CCD, etc., are conventionally known. These cameras have drawn attention in recent years in particular as input devices to input images into image processors such as personal computers.
It would be very convenient if interchangeable lenses for single lens reflex cameras, which already exist in great numbers corresponding to various incident angles of view, could be employed as image forming lenses for these types of cameras. However, due to differences between the size of the image formed on the surface of the silver halide film and the size of the image formed on the surface of an electronic imaging device (e.g., a CCD, etc.), interchangeable lenses for single lens reflex cameras can not be directly used as image forming lenses for video cameras, etc. Therefore, where an interchangeable lens for single lens reflex cameras is used, a relay lens system must be used to reduce the image formed by the interchangeable lens and re-form it onto the CCD.
An optical system using this type of relay lens system is explained with reference to FIGS. 4 and 5. FIG. 4 is a simplified drawing showing the basic construction of an optical system employing a relay lens system, and FIG. 5 is a drawing of the optical path of an SVC using the optical system of FIG. 4.
In FIGS. 4 and 5, L1 is an interchangeable lens (image forming lens) for single lens reflex cameras, I1 is a first image plane, L2 is a field lens, L3 is a relay lens system, and I2 is a second image plane. In FIG. 5, 101 is a surface onto which the image forming lens L1 is mounted, and 102 and 103 are reflecting mirrors. In FIGS. 4 and 5, the light rays leaving the image forming lens L1 form an image at the first image plane I1. The light rays that formed the first image then strike the field lens L2. The field lens L2 is a lens located near the first image plane I1 and has a positive refractive power. In addition to causing the light rays that formed the first image to converge toward the optical axis, the field lens L2 causes the light rays to strike the relay lens system L3. In other words, the field lens L2 is located such that the relay lens system L3 may be used with approximately the same angle of view as the image forming lens L1, i.e., the field lens L2 operates to coordinate the exit pupil of the image forming lens L1 with the entry pupil of the relay lens system L3. The light rays that are made to converge toward the optical axis by means of this field lens L2 are reduced and projected onto the second image plane I2 by the relay lens system L3 and form a second image.
However, this type of conventional relay lens system has problems. Generally, the position along the optical axis and size of the exit pupil of the image forming lens that forms the first image vary depending on the focal length and the F-number of the image forming lens. In addition, because the positions of the principal point and the exit pupil along the optical axis do not necessarily coincide, the position of the exit pupil may differ even where the focal length is the same.
Therefore, where the image forming lens L1 is replaced with a lens having a short focal length and a large exit pupil, there are cases where non-axial or marginal rays do not strike the relay lens system L3, as shown in FIG. 6. In such a case, i.e., where the exit pupil of the image forming lens and the entry pupil of the relay lens system do not substantially coincide, the amount of non-axial rays is reduced and the peripheral areas of the second image become dark. Therefore, in a conventional relay lens system having a single entry pupil, only image forming lenses whose exit pupil substantially coincides with the entry pupil of the relay lens system can be used.
To resolve this problem, it is possible to form a second image without the use of non-axial rays that do not strike the relay lens system. However, in this case, the size of the second image that can be transferred from the first image becomes smaller, which is not desirable.
It is also possible to give the field lens a variable refractive power. However, in order to do this, the large field lens would have to be moved, which would increase the size of the entire optical system. Moreover, if the field lens were moved, fluctuations in its aberrations would affect the image, which is not desirable.